Stabilization of refrigeration centrifugal compressor



Dec- 5, 1961 E. w. TANZBERGER ETA]. 3,011,322

STABILIZATION OF REFRIGERATION CENTRIFUGAL COMPRESSOR Filed Aug. 12,1958 2 Sheets-Sheet 1 lSd GU (ISSEIZid ISd GHOSSHEH INVENTORS EJJC WTanzberger, BY Chaflesffiman,

ATTORNEYS Dec. 5, 1961 E. w. TANZBERGER ET AL 3,011,322

STABILIZATION OF REFRIGERATION CENTRIFUGAL COMPRESSOR Filed Aug. 12,1958 2 Sheets-Sheet 2 IN VENTORS Er] Tanzbarger, $0110.95. dram, WATTORNEYS.

United States Patent @fifice 3,011,322 Patented Dec. 5, 1961 3,011,322STABILIZATION F REFRIGERATION CENTRIFUGAL COMPRESSOR Eric W. Tanzberger,Clean, and Charles E. Green, Allegany, N .Y.,assignors to DresserOperations, Inc., Whit- 1 tier, Calif., a corporation of CaliforniaFiled Aug. 12, 1958, Ser. No. 754,559

- 6 Claims. (Cl. 62-196) This invention relates to the stabilization ofa centrifugal compressor used in a refrigeration system.

As is well known, a centrifugal compressor can surge when under lightload.

The main object of the present invention is to improve the stability ofacornpressor of the centrifugal type when used in a refrigeration systemwhere the demand for refrigeration varies and particularly when suchdemand is low. A reduction in capacity up to 90% has been made withoutsurge occurring in the compressor of the present invention.

, in accordance with the present invention a centrifugalcompressor forcompressing refrigerant gas and having two .or more stages isincorporated in a refrigeration system including a condenser, flashcooler or economizer, and evaporator, in such a way as to stabilize thecompressor under light load. This is accomplished by arranging thecompressor in series with the condenser flash cooler and evaporator andproviding a by-pass connection between the flash cooler and the inlet ofthe second stage and incorporating means to control the amount of gasfed to the second stage, whereby under normal load or full capacity thesecond stage receives gas jointly from the outlet of the first stage andthe flash cooler but under low load substantially all of the output ofthe first stage is by-passed to the flash cooler so as to be recycledthrough the first stage. The portion of gas not so by-passed is expandedinto the second stage to provide sufficient volume to keep the secondstage and any subsequent stages out of surge. The means controlling thesecond stage inlet are provided preferably by movable guide vanesadjusted automatically- Other objects and advantages of the presentinvention will be apparent from the following description andaccompanying drawings in which: FIG. 1 is a diagrammatic representationof the various components, including the new compressor of the presentinvention, forming the refrigeration system. FIG. 2 is a Mollier typediagram of the refrigerant cycle for the system when under normal orfull load.

FIG. 3 is a similar diagram for the system when under a light loadrepresenting 10% capacity.

FIG. 4 is an enlarged vertical central sectional view through the secondstage of the improved compressor shown in FIG. 1. I 1 FIG. 5 is afragmentary vertical transverse sectional view thereof, taken on line 55of FIG. 4, and showing certainportions broken away to reveal other partsmore clearly. 1

' FIG. 6 is a fragmentary sectional view of a modified and hand controlmeans for adjusting the movable guide vanes.

Referring to FIG. 1, the numeral 10 represents a condenser of anysuitable form, the same being shown as of the shell-and-tube type inwhich cooling water flows through the tubing 11 and the refrigerant isin the shell outside the tubes. The shell of the condenser is shown asformed with a sump in which the condensed liquid refrigerant collectsand from which sump the condensate can flow by gravity under control ofa float control valve 12. The outlet of the condenser 10 is connected bya,

conduit 13 to a flash cooler or economizer 14 having an outlet in itsbottom through which liquid refrigerant can flow by gravity undercontrol of a fioat control valve 15. The aforesaid outlet of the cooler14 is connected by a conduit 16 to the bottom of an evaporator 1'8 whichis also shown as being of the shell-and-tube type in which the secondaryrefrigerant liquid such as water or brine flows through the tubing 19and the refrigerant is in the shell outside this tubing, The secondaryrefrigerant liquid circulates between the evaporator 18 and the coolingload served by the refrigeration apparatus to transfer heat from theload to the evaporator 18. In removing heat from the secondaryrefrigerant, the refrigerant in the evaporator 13 boils forming gaswhich passes through the liquid eliminator 20 and thence into thesuction pipe 21 to the inlet of a multiple stage centrifugal compressor,indicated generally at 22.

The compressor 22 is shown as having first and second stages I and IIseparated by a hermetically sealed water cooled electric drive motor 23of conventional form. The impellers 24, and 25 of the first and secondstages, respectively, of the centrifugal compressor 22 are rotativelydriven by the motor 23. The impeller 24 of the first stage is arrangedin a casing 26 having an inlet 28 and outlet 29. The impeller 25 of thesecond stage is arranged in a casing 30 having an inlet 31 and outlet32. The inlet 31 communicates with an inlet volute 27 forming part ofthe casing 30 and leading to the annular inlet mouth for the impeller25. The suction pipe 21 of the evaporator 18 is connected to the inlet28 of the first stage. The outlet 29 of the first stage is connected tothe inlet 31 of the second stage by a crossover conduit 33. A by-passconduit 34 is shown as connecting the cross-over conduit 33 to theinterior of the cooler 14, entering in the top wall thereof. The outlet32 of the second stage is connected to the condenser 10 by a conduit 35.

Thus it will be seen that the condenser 10, cooler 14, evaporator 18 andthe multiple stage centrifugal compressor 22 are connected in seriesthrough which the refrigerant cycle but with a by-pass 34 being providedbetween the cooler 14 and the cross-over conduit 33 intermediate thefirst two stages of the compressor.

Under normal or full load the refrigerant cycle is as depicted in theMollier type diagram of FIG. 2. Letters have been used to indicatedifferent points on the diagram and these letters have also been appliedto FIG. 1. Referring to FIG. 2, S represents the saturation line betweenliquid and gas and is the characteristic curve for the particularrefrigerant under consideration which may be of any suitable type suchas one of the Freons. At point A the refrigerant gas enters the inlet ofthe first stage I of the compressor and leaves at point B having ahigher pressure and heat content. In passing through the crossover pipe33 from the outlet of the first stage (point B) to the inlet of thesecond stage (point C) the gas loses a slight amount of heat. In passingthrough the second stage of the compressor the gas in further-compressedand heated to the point D which is representative of the condition ofthe gas at the outlet of the second stage. The gas then passes throughthe line 35 and condenser 10 in which it is condensed to point E, thisrequiring the Inasmuch as removal of heat without loss of pressure.

the bypass line 34 is at a lower pressure than the line 35 and condenser10, the liquid refrigerant leaving point B is partially evaporated,thegas generated moving through the by-pass conduit 34 to point Pindicated on the diagram. The portion of the liquid refrigerantremaining liquid passes through the flash cooler withsome loss of heatto point G. In moving from point G to point A; completing the cycle, theliquid refrigerant is heated and loses pressure. On the diagram the lineconnecting points G and A represent the refrigerant effect or coolingload.

In accordance with the principles of the present invention, the amountof refrigerant gas entering the second stage of the compressor isreduced as the refrigeration load decreases. Assuming that the load hasdecreased to of normal capacity, the refrigeration cycle depicted by thediagram of FIG. 3 obtains. A reduction in refrigeration or cooling loadis effected by a reduction of the temperature differential between theinlet and outlet of the tubing 19 in the evaporator 18.

Referring to the diagram of FIG. 3, at point A the refrigerant gasenters the first stage of the compressor and leaves at point B. Some ofthe initially compressed gas leaving point B is by-passed through theconduit 34 to the cooler 14, as depicted at point P. The remainingportion of the first stage output gas flows through the cross-overconduit 33 and enters the inlet of the casing for the second stage.Arranged within this casing are means, later to be described, forthrottling the entering gas. As a result of being throttled, thepressure of the gas is reduced to point C. In passing through the secondstage of the compressor the gas is further compressed and increases inheat content to point D. Thereafter the gas flows through the conduit 35and condenser 10 with loss of heat to point E. The gas entering thecooler 14 through the by-pass conduit 34 is mixed with the liquidrefrigerant from the sump of the condenser and then moves to the outletof the cooler 14 to point G. The reduced refrigeration or cooling loadis only diagrammatically represented by the line connecting the points Gand A in FIG. 3.

Thus it will be seen that under reduced load conditions a portion of theoutput of the first stage is by-passed around the second stage andcondenser and returns to the flash cooler 14. Sutficient gas, however,is allowed to enter the inlet of the second stage so that after suchentering gas is throttled it is expanded in volume sufiiciently to keepthis second stage from surging.

While any suitable means may be employed for controlling the amount ofgas entering the second stage, the means preferred and illustratedinclude movable inlet guide vanes, indicated at 40. Referring to FIG. 5,a plurality of such movable guide vanes are arranged within the inletvolute 27 in circular fashion at uniform circumferiential intervalsabout the inlet mouth for the impeller 25 of the second stage. Eachguide vane 49 is shown as being of air-foil shape in cross-section andas mounted non-rotatably on a stub shaft 41 having an axis parallel tothat of the impeller 25. Fast to each of these shafts 41 is a piniongear 42. All of the pinion gears 42 are engaged by an externally toothedring gear 43 as best shown in FIG. 4. Each of the shafts 41 is suitablyjournalled on either side of its pinion gear 42 within the volute 27.The ring gear 43 is also suitably journalled for free rotation withinthe volute 27. The shaft for the uppermost guide vane is shown as beinglongitudinally extended as indicated at 44 to extend outwardly throughthe side wall of the inlet volute 27.

It will be seen that if the extended shaft 44 is rotated the pinion 42fast thereto will also rotate, causing the ring gear 43 to rotate. Sincethe teeth of the ring gear 43 engage the teeth of all the other piniongears 42, the latter are likewise caused to rotate, all resulting in thevanes being rotated about their axes in the same direction and throughthe same angular displacement, for a given angular motion imparted tothe master or extended shaft 44.

If desired, stationary guide vanes 37 may be arranged in the volute 27radially outwardly of the movable inlet guide vanes 40, as shown in FIG.5. The movable inlet guide vanes 40 as represented in full lines in FIG.5 illustrate the positions of these vanes under normal load or fullcapacity. It will be seen that the space or passage 45 between twoadjacent vanes 40 is relatively large. However, if these vanes 40 areadjusted, by turning the shaft 44, they can be caused to assume thedotted line positions shown in FIG. 5 where the space or passage betweenadjacent vanes, as indicated at 46, is relatively narrow. This ratherclose spacing of the vanes represents their position when therefrigeration load is light. Less gas enters through the restrictedpassages 46 than through the larger passages 45 when the vanes are openfully, and in passing through the restricted passages 46 the gas isthrottled and expanded in volume, of course, with an attending reductionin pressure. However, the expanded volume is sufficient to keep thesecond stage of the compressor from surging.

Any suitable means may be employed for controlling the adjustment of themovable inlet guide vanes 40. The automatic means shown in FIGS. 1 and 4are preferred, although the manual means shown in FIG. 6 may be used, ifdesired.

Referring to FIG. 4, the extended shaft 44 extends laterally outwardlythrough the outer side wall of the inlet volute 27, being suitablysealed with respect thereto. The outer end of this shaft has fastthereto a pinion gear 48 which engages with a vertically movable gearrack 49. The rack 49 is shown as being guided for rectilinear verticalmovement by the guide 50 arranged within the box 51 mounted on the outerside wall of the volute 27. Depending from the lower end of the gearrack 49 is an actuating rod 52, which extends downwardly through anopening provided in the bottom of the box 51. The lower extremity of therod 52 is externally threaded to receive a cylindrical slide member 53received in a tubular extension 54 attached to the bottom of the box 51and extending downwardly therefrom.

The exit of the actuating rod 52 from the box or housing 51 is sealedagainst leakage of gas. For this purpose, a bellows 55 surrounds the rod52 within the box 51. At its upper end the bellows 55 is connected to acollar 56 welded to the rod, and at its lower end the bellows isconnected to a sleeve 57 surrounding the rod 52 in spaced relationthereto. The lower end of the sleeve 57 is welded to the bottom of thebox 51. In this manner, the rod 52 can move in and out of the box 51with at constant seal being maintained.

Referring to FIG. 1, slide member 53 is connected to the piston rod of apiston and cylinder device 60' which may be of any suitableconstruction. Such piston and cylinder device 66 has a suitableconnection with a supply of pressurized fluid (not shown) and is shownas having a solenoid valve 61 to control the direction and extent ofmovement of the piston rod of the device 60. In order to render theoperation of the piston and cylinder device responsive to thetemperature of the secondary refrigerant passing through the tubing 19of the evaporator 18, a suitable temperature sensing device 62, such asa thermocouple unit, is shown as being operatively associated with theinlet end of the tubing 19 and connected by the electrical control lines63 to the solenoid control valve 61.

In this manner, the position of the movable inlet guide vanes 40 can beautomatically adjusted in direct response to the refrigeration load assensed from the secondary refrigerant passing through the tubing 19. Forexample, if the temperature of the secondary refrigerant passing throughthe leg of the tubing 19 in which the temperature sensing device 62 isarranged, is relatively high, indicating a refrigeration demand, thesolenoid valve 61 is actuated so as to cause the piston and cylinderdevice 60 to operate to move the actuating rod 52 and hence gear rack 49upwardly. This rotates the pinion 48 in a clockwise direction as viewedin FIG. 5, thereby rotating the extended shaft 44 likewise in aclockwise direction. The pinion 42 on the extended shaft 44 is similarlycaused to rotate in a clockwise direction, but by reason of itsengagement with the ring gear 43, the latter is caused to move in acounter-clockwise direction. This in turn causes the other pinions 42and the various other guide vanes 40 to rotate about their respectiveaxes in a clockwise direction to a more fully open position as showntypically by full lines in FIG. 5.

However, assume now the temperature of the secondary refrigerantentering the tubing 19 lowers in value, indicating a decrease inrefrigeration demand. The temperature sensing device 62 sends a commandsignal to the solenoid of the solenoid control valve 61 thereby causingthe piston and cylinder device 60 to pull down on the actuating rod 52and hence also move the gear rack 49 downwardly. This causes a reverseddirection of rotation of the various pinions 42. They now move in acounter-clockwise direction so as to close the gaps between adjacentguide vanes 40, for example, to the restricted spacing 46 shown in FIG.5.

The restricting of the passages through which the re- 7 frigerant gasentering the second stage of the compressor must pass changes therefrigeration cycle to that typically depicted in FIG. 3. Under suchconditions of operation some of the output of the first stage of thecompressor is by-passed back to the flash cooler 14 to berecycledthrough this cooler and evaporator 18, the gas not so by-passedpassing through the restricted passages, as illustrated by the space 46in FIG. 5. "In passing through these spaces the gas is throttled orexpanded. This has the effect of increasing its volume although loweringits pressure which operates to keep the second stage of the compressor,and any additional stages that may be employed'in the multi-stagecompressor, from surging.

Instead of the automatic means for controlling adjustment of the movableinlet guide vanes 40 shown in FIGS. 1 and 5, the manually adjustingmeans shown in-FIG. 6 may be employed. The only modification made is theelimination of the slide member 53 and associated apparatus 60-63,cylinder member 54, and the substitution of the hand wheel means shownin FIG. 6. As

there shown, a hand wheel-66 is mounted on a hub 67 internally threadedto receive the external thread on the lower end portion of the actuatingrod 52. The hub 67 is shown as being suitably rotatably mounted on thebottom of the box 51 but in such manner. as to prevent any axialdisplacement of this hub. It will be seen that by turning the hand Wheel66 in one direction the actuating rod 52 and also the gear rack 49 towhich this rod is connected, are moved in one direction. When the handwheel is turned in the opposite direction the actuatingrod 52 and gearrack 49 moves in the opposite direction. v

It is an important feature of the present'invention to 7 note that thereare no flow control means, such as guide vanes, at the inlet to thefirst stage of the compressor. The flow control means, such as themovable inlet guide vanes 40, are provided only at the inlet to thesecond stage ofthe compressor. Such an arrangement, including theby-pass conduit 34 interposed between the cross over conduit 33 betweenthe first two stages of the compressor and the flash cooler 14, has beenfound in actual practice to the highly effective inpreventing thecompressor from surging under light load conditions, even to the extentof a reduction of 90% of load capacity.

We claim: 1. In a refrigeration system having a condenser, flash coolerand evaporator connected in series, the combination therewith of acentrifugal compressor having at least two stages, the inlet of thefirst stage being connected to said evaporator, the outlet of the laststage being connected to said condenser, a first conduit connecting theoutlet of the first stage with the inlet offlthe second stage, a secondconduit connecting said first conduit with said cooler, and flow controlmeans only for the second one of the stages and arranged downstream ofthe connection between said conduits but upstream of the inlet to thesecond stage to throttle refrigerant gas entering the second stagethereby to stabilize .the compressor under light load.

2. In a refrigeration system having a condenser, flash- 7 two stages,the inlet of the first stage being connected cooler and evaporatorconnected in series, the combinatron therewith of a centrifugalcompressor having at least to said evaporator,'the outlet of the laststage being connected to said condenser, a first conduit connecting theoutlet of the first stage with the inlet of the second stage, a secondconduit connecting said first conduit with said cooler, movable inletguide vane means only for the second one of the stages and arranged tocontrol the flow of refrigerant gas into the second stage, and controlmeans arranged to move said guide vane means to throttle the refrigerantgas entering the second stage when the compressor is under light loadthereby to stabition therewith of a centrifugal compressor having atleast two stages, the inlet of the first stage being connected to saidevaporator, the outlet of the last'stagebeing connected to saidcondenser, a first conduit connecting the outlet of the first stage withthe inlet of the second stage,

a second conduit connecting said first conduit with said cooler, movableinlet guide vane means only for the second one of the stages andarranged to control the flow of refrigerant gas into the second stage,and automatic control means responsive to refrigeration load variationand arranged to move said guide vane means to throttle the refrigerantgas as the refrigeration load decreases and vice versa, thereby tostabilize the compressor.

5. In a refrigeration system having a condenser, flash cooler andevaporator connected in series, the combination therewith of acentrifugal compressor having at least two stages, the inlet of thefirst stage being connected to I said'evaporator, the outlet of the laststage being connected to said condenser, a first conduit connecting theoutlet of the first stage with the inlet of the second stage, a secondconduit connecting said first conduit with said cooler, movable inletguide vane means only for the second one of the stages and arranged tocontrol the flow of refrigerant gas into the second stage, and automaticcontrol means responsive to refrigeration load I variation and arrangedto move said guide vane means flow control means only for the second oneof the stages and arranged upstream of the inlet to the second stage tothrottle the refrigerant gas entering the second. stage and receivedfrom the first stage, and means arranged upstream of said flow controlmeans for bleeding-off some of the refrigerant gas discharged by thefirst stage for recirculation through the first stage.

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